Method and device for drilling holes in workpieces by means of laser beams

ABSTRACT

A method and a device for producing holes in workpieces by using at least one laser beam, in particular a short-pulse or ultrashort-pulse laser beam, whereby a process gas is supplied to the point of contact of the laser beam with the workpiece. At least one parameter of the process gas, i.e., its composition, pressure, volume flow, for example, is selected and the process gas is supplied to the interaction zone between the laser beam and the workpiece in such a way that the hole produced by the laser beam has a desired quality (hole geometry, reduced ablation residues, or none at all, etc.).

FIELD OF THE INVENTION

The present invention relates to a method and a device for producingholes in workpieces by using at least one laser beam.

BACKGROUND INFORMATION

Conventional devices and methods are used for producing precisemicroholes in workpieces by using laser beams, where the holes may havea diameter smaller than 250 μm.

In laser beam drilling, in particular using short-pulse lasers with theconventional devices, melt burrs are formed in workpieces made of ametallic material on the laser beam entrance side of the workpiece whichmust be deburred in an afterworking operation. Furthermore, a melt filmis formed in the hole, i.e., on the peripheral surface of the hole,which must also be removed subsequently, but there must not be anyunwanted loss of material at the edges of the hole and/or blockage ofthe hole.

In the field of fuel injection, for example, there is a growing demandfor conical holes, e.g., in nozzles, so that the laser beam entranceopening (fuel outlet) usually requires a smaller diameter than the laserbeam exit opening (fuel inlet). Conventionally, to produce these holeshaving a desired conicity, the workpiece may be tilted in relation tothe laser beam and/or the machining strategy, i.e., the parameters ofthe laser beam, may be adjusted accordingly. It has been found that theconicity which is achievable with the conventional methods and isdefinable on the basis of the k factor is relatively limited, dependingon the diameter of the hole.

SUMMARY

A method according to an example embodiment of the present invention mayhave the advantage over the related art that it is possible to produceholes of the desired quality through controlled adjustment of at leastone process gas parameter, in particular the composition, pressure,and/or volume flow of the process gas and a special process gas supply.It is not necessary here to change the laser beam parameters or to tiltthe workpiece with respect to the laser beam. In conjunction with thepresent invention, at least the following criteria determine the“quality” of the hole: the shape of the hole and—in the case ofworkpieces made of at least one metallic material—the ablation residues.Ablation residues are understood in particular to refer to melt burrs(“piles of melt”) on the workpiece surface on the laser beam entranceside and the melt film on the wall or walls of the hole. It has beenfound that melt burrs may be prevented completely by the methodaccording to the present invention, and the resulting melt film has onlya very small thickness. Ablation residues may thus be prevented orreduced at least to a harmless extent, so that afterworking of the holemay be reduced, i.e., simplified or possibly eliminated entirely. Theexample method according to the present invention also permits acontrolled influence on the shape of the hole. Holes having a circularcross section that is constant or generally constant over the entirelength of the hole as well as holes having a desired conicity may beimplemented. The “conical” holes preferably have a circular crosssection with the diameter varying over the length of the hole. Due tothe fact that at least the hole shapes mentioned above are implementableby this method, ablation residues are prevented and/or significantlyreduced, so inexpensive production of one or more holes is readilypossible. Furthermore, it has been found that the machining times may beshortened in comparison with conventional methods.

The principle on which the example method according to the presentinvention is based is itself based on the fact that the process gassupplied to the interaction zone between the laser beam and theworkpiece determines the properties of the material vapor-plasma mixtureand thus the interaction between the laser beam and the workpiece. Theatmosphere surrounding the hole is compressed due to materialevaporating from the workpiece, so a strong shock wave may develop andmay reach a rate of propagation of up to several times 10 km/s, e.g.,when machining a workpiece by using a short-pulse laser. The shock waveforms a barrier for the material evaporating from the workpiece, withthe pressure, the density, and the temperature and thus also the degreeof ionization and the absorption capacity in the material vapor-plasmamixture being related to the properties of the shock wave. The rate ofpropagation of the shock wave and its thermodynamic properties are alsoa function of the atmosphere surrounding the hole. By the methodaccording to the present invention, it is possible to create atmosphericconditions around the hole such that the advantages mentioned above maybe achieved.

An example embodiment of the method that is particularly preferred ischaracterized in that preferably for each specific application, thecomposition, pressure, and/or volume flow of the process gas supplied tothe interaction zone and/or the supply strategy (process gas management)are adjusted as a function of at least one characteristic feature of thehole, e.g., the hole diameter, the desired conicity, a defined roundingof at least one of the hole rims, i.e., edges and the like, and/or atleast one characteristic feature of the workpiece, e.g., the wallthickness, the workpiece material, etc. The parameters of the processgas during the production of a hole need not necessarily be constant butinstead may also be controlled—as provided in an advantageousvariant—preferably over time. Thus, for example, “preboring” of the holeusing a process gas composed of helium and a subsequent“afterboring/boring” using a process gas composed of argon are bothpossible.

In one example embodiment of the method, the laser beam is a short-pulselaser beam (ns pulses) having a pulse duration of preferably less than100 ns or an ultrashort-pulse laser beam (fs/ps pulses). Other laserconcepts may of course also be used to implement this method.

In one example embodiment, a hole having a desired conicity is produced,the conicity factor (k) of the hole being variable through appropriateprocess gas management and adjustment of the process gas parameters. Ithas been found that holes having a conicity factor k of −3 to +3 areeasily produced. However, the method according to the present inventionalso makes it possible to achieve conicity factors greater than +3.Conicity factor k is defined as follows:(ø_(A)−ø_(E))/10where ø_(A) is the diameter of the laser exit opening and ø_(E) is thediameter of the laser entrance opening, and the dimensions of thediameter are given in μm (micrometers).

By the example method according to the present invention, practicallyany desired conicity may be achieved for holes having a diameter of lessthan 250 μm and for wall thicknesses of the workpiece of approximately0.2 mm to 2 mm, where conicity factor k may be in a range from −3 to +3,for example.

The at least one process gas used during the production contains atleast one gas, for example, helium (He), oxygen (O₂), argon (Ar), ornitrogen (N₂). However, the process gas may also contain multiple gases,in particular those mentioned above, which are mixed together preferablybefore being supplied to the interaction zone, i.e., before reaching theinteraction zone. If the process gas is combined from multiple gases,the amount of each gas in the process gas may be between 0% and 100%,with the sum of the proportions of all gases of the process gasamounting to 100%. Since different gases have different physicalproperties, the process gas is composed of different gases in a certainmixing ratio, so that a desired atmosphere may be achieved around thehole to be drilled by using the laser beam, resulting in a desired holeshape, e.g., conicity of the hole, and preferably minimizing oreliminating melt burrs and the melt layer on the walls of the hole inlaser machining of metallic materials.

It has proven advantageous if the volume flow supplied to theinteraction zone is in a range from approximately 0.8 Nl/min to 270Nl/min and if the pressure of the process gas is in a range from 0.1 barto 20 bar, preferably from 0.3 bar to 15 bar, in particular from 0.5 barto 10 bar.

One example device according to the present invention includes a laserbeam source for generating at least one laser beam directable at theworkpiece and a nozzle configuration having at least one nozzle whichmay receive at least one pressurized process gas, the gas stream emittedfrom the nozzle being directable in the direction of the point ofcontact of the laser beam with the workpiece and/or into the interactionzone between the laser beam and the workpiece. This device ischaracterized by an apparatus for adjusting the composition, pressure,and/or amount of process gas supplied to the point of contact. By usinga process gas suitable for the particular application and/or thetargeted adjustment of the process gas pressure and/or volume flow, itis possible to vary and/or adjust the conicity of holes in a controlledmanner without having to adjust the parameters of the laser beam.Furthermore, in the case of metallic workpieces, melt burrs and/or meltfilms on the wall of the hole may be minimized, preferably eliminatedentirely.

According to one refinement of the present invention, the example devicehas at least one mixing apparatus for mixing the process gas and atleast one supply line in fluid connection to the nozzle for supplyingthe mixed process gas. The process gas is thus combined before reachingthe interaction zone between the laser beam and the workpiece. Themixing apparatus preferably has a controller for adjusting the processgas parameters (composition, pressure, volume flow).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained in greater detail below in severalexemplary embodiments with reference to the drawings.

FIG. 1 shows a basic diagram of the design of an exemplary embodiment ofthe device according to the present invention.

FIGS. 2-4 each show an exemplary embodiment of a nozzle configurationfor supply process gas.

FIGS. 5 and 6 each show a cross section through a hole produced by usinga laser beam and using various process gases.

FIG. 7 shows a diagram in which the diameter of the hole on the laserbeam entrance side and the conicity factor k are plotted as a functionof the process gas pressure.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows a schematic diagram of the design of an exemplaryembodiment of a device 1 for creating holes in workpieces. Device 1includes a laser beam source 3 by which short-pulse laser beams orultrashort-pulse laser beams may be generated, these beams beingreferred to below simply as laser beams 5. An expanding lens 7, atrepanning lens 9 and a focusing lens 11 are situated in the path of thebeam of the laser. In the area between trepanning lens 9 and focusinglens 11, laser beam 5 is deflected by 90°, for example, by a deflectingreflector 13. In another exemplary embodiment (not shown in the figures)of device 1, no expanding lens 7 is situated in the path of the beam.

FIG. 1 shows a workpiece 15 onto which laser beam 5 is directed to drilla hole, in particular a through-hole, having precision conicity.Workpiece 15 is situated downstream from the focusing lens, as seen inthe direction of the path of laser beam 5. A control device (not shown)is provided for positioning workpiece 15 so that the position ofworkpiece 15 where the hole is to be drilled is situated exactly in thepath of laser beam 5, workpiece 15 being adjustable by this device indirections x, y and z, as indicated by arrows.

Device 1 also has an apparatus 16 for adjusting the composition of theprocess gas, this apparatus including a mixing device 17 having acontroller for mixing the process gas. The process gas preferablycontains at least one of the following gases or a mixture thereof:helium, oxygen, argon, and nitrogen. These gases and, if necessary,other suitable gases are mixed together in a certain preselectable ratioin mixing apparatus 17. If the process gas is a mixed gas, the amount ofeach gas in the process gas may be between 0% and 100%. The process gasvolume flow supplied to the interaction zone between laser beam 5 andworkpiece 15 is preferably in a range from approximately 0.8 Nl/min to270 Nl/min (standard liters per minute). The process gas pressure ispreferably between 0.1 bar and 20 bar, in particular between 0.5 bar and10 bar.

The process gas is supplied through a supply line 19 to a nozzleconfiguration (not shown in FIG. 1) which includes at least one nozzle.With the help of apparatus 16, the pressure and/or the quantity ofprocess gas supplied to the interaction zone between laser beam 5 andworkpiece 15 may be adjusted. Control of process gas parameters overtime is easily implementable. For example, preboring of the hole usinghelium and then finish-boring with argon are possible.

FIG. 2 shows a schematic diagram of a detail of a first exemplaryembodiment of nozzle configuration 21, which includes a nozzle 23 towhich pressurized process gas may be supplied. Nozzle 23 has a conicalshape in the longitudinal section, its cross section through whichprocess gas flows becoming smaller in the direction of workpiece 15. Theconfiguration of nozzle 23 here is selected so that process gas stream25 emitted from nozzle 23 runs coaxially with laser beam 5. Process gasstream 25 and laser beam 5 are aligned here perpendicular to a top side27 of the workpiece.

FIG. 3 shows a second exemplary embodiment of nozzle configuration 21which differs from nozzle configuration 21 described with reference toFIG. 2 only in that nozzle 23 is aligned in relation to laser beam 5 sothat process gas stream 25 emitted from nozzle 23 is supplied to laserbeam 5 and/or the interaction zone at an angle α of approximately 90°.Process gas stream 25 here runs parallel to planar workpiece top side27. Nozzle 23 is preferably designed to be adjustable, so that thesetting of angle α at which process gas stream 25 runs in relation tolaser beam 5 is adjustable, namely between 0° and 90°.

FIG. 4 shows a third exemplary embodiment of nozzle configuration 21having nozzles 23A and 23B, where nozzle 23A corresponds inconfiguration and design to nozzle 23, which was described withreference to FIG. 2, and nozzle 23B corresponds to nozzle 23, which wasdescribed with reference to FIG. 3. As indicated with a double arrow,nozzle 23B is adjustable in relation to nozzles 23A, so that angle α atwhich process gas streams 25, i.e., process gas 25 blown out of nozzles23B, and laser beam 5 strike one another is variable between 0° and 90°.Nozzles 23A, 23B may receive the same process gas or different processgases. The parameters of the process gas streams emitted from nozzles23A, 23B are preferably independently adjustable, thus permittingoptimum adjustment of the atmosphere around the hole created inworkpiece 15 by laser beam 5.

As an alternative to nozzles having a conical cross section as describedwith reference to the figures, for example, Laval nozzles, ring nozzles,free forms, or similar types may also be used, i.e., the nozzle geometrydescribed above is only one of many possible nozzle geometries.

With device 1, which is described with reference to FIGS. 1 through 4,the method according to the present invention may be readilyimplemented. It provides for the composition, the pressure, and/orvolume flow of process gas to be selected and for the process gas to besupplied to the interaction zone between the laser beam and theworkpiece in such a way that the hole produced by the laser beam has adesired quality, in particular conicity and/or little or no melt burrsor melt films without workpiece 15 wobbling and/or without having toalter the parameters of laser beam 5. The device parts mentioned aboveare preferably situated in a stationary position with respect toworkpiece 15, at least during the production of the bore. This isunderstood not to include the optical wedge plates of trepanning lens 9indicated in FIG. 1, which are in rotation during the operation ofdevice 1.

FIG. 5 shows a longitudinal section through a conical hole 29 producedby device 1, its largest diameter being approximately 100 μm. ThicknessD of workpiece 15 is relatively small and may easily be in a rangebetween 0.2 mm and 2.0 mm. The process gas supplied to the interactionzone between laser beam 5 and workpiece 15 during the production of hole29 consists of 80% argon and 20% helium. Hole 29 has a diameter ø_(E) atlaser beam entrance opening 31 which is greater than diameter ø_(A) atlaser beam exit opening 33. Conicity factor k ((ø_(A)−ø_(E))/10) hereamounts to approximately −2. Hole 29 has practically no melt burrs andthe melt film (not shown) on the wall of the hole/bore has a minimalthickness. The edges of the hole at the point of laser entrance and exithave relatively sharp edges. Rounded hole edges may also be implementedthrough an appropriate change in the process gas parameters and/or thecomposition of the process gas.

FIG. 6 shows a longitudinal section through a conical hole 29, in theproduction of which the process gas supplied to the interaction zonebetween laser beam 5 and workpiece 15 consists of 20% argon and 80%helium. The other process gas parameters (pressure, volume flow) and thelaser beam parameters are the same as those in the production of hole 29shown in FIG. 5. It is readily apparent that hole 29 has a much largerconicity factor k, amounting here to approximately −1. This means thatthe conicity of hole 29 is different only due to the change in thecomposition of the process gas, i.e., the size of the amount by volumeof the gases added to the process gas. The conicity of the hole may thusbe varied in a controlled manner determined exclusively by thecomposition of the process gas. In addition, a controlled supply of theprocess gas to the interaction zone is necessary, as described withreference to FIGS. 1 through 4, for example. To permit an even moreprecise setting of the conicity of the hole, in this variant thepressure and the process gas volume flow supplied to the interactionzone between laser beam 5 and workpiece 15 may also be variedaccordingly.

The hole shown in FIG. 6 has melt burrs 35 on workpiece top side 27, incontrast with hole 29 shown in FIG. 5, while the edge of the hole atlaser beam exit opening 33 has sharp edges. Through appropriateadjustment of the process gas parameters and controlled process gassupply to the interaction zone, the characteristic features of the hole(conicity, diameter, sharp edges or rounded, etc.) may also becontrolled accurately.

In the holes having a diameter smaller than 250 μm and a wall thicknessD of workpiece 15 less than or equal to 2 mm, any desired conicityfactor (k) in the range between −3 and +3 or even larger may easily beimplemented by selecting a corresponding process gas composition and bycontrolled supply of the process gas to the interaction zone. Otherparameters for adjusting a precise conicity of the hole include thepressure and volume flow of the process gas.

In an advantageous exemplary embodiment of the device according to thepresent invention, an optical apparatus, in particular having a specialtrepanning lens, is used to influence the laser beam by which alone itis possible to produce conical holes having a certain conicity factor,preferably adjustable, without having to vary, i.e., adjust, the processgas parameters and management in a special way. For example, a conicalhole having a conicity factor of 5 is producible with a certain settingof the lens. By the method according to the present invention whichprovides for targeted influencing of at least one process gas parameterand a special supply of process gas to the interaction zone, it is nowpossible in an advantageous manner to increase and decrease thisconicity factor of 5, preferably with a high precision, e.g., to 5.4 or3.7 or 7.8. In other words, a precision adjustment of the conicity ofthe hole which is implemented, i.e., determined, by the laser beamparameters is possible without having to alter the laser beamparameters.

FIG. 7 shows a diagram in which hole diameter ø_(E) in μm (micrometers)on the laser beam entrance side is plotted on the left ordinate,conicity factor k of hole 29 is plotted on the right ordinate andpressure p of the process gas in bar, where the process gas is composedof 50% helium and 50% oxygen, is plotted on the abscissa. Severalmeasured hole diameters ø_(E) and a particular conicity factor k areplotted in the diagram as a function of the process gas pressure. Theholes were all produced under the same conditions, i.e., the laser beamparameters and the composition of the process gas were the same;likewise, the manner in which the gas was supplied to the interactionzone was also the same. Only the pressure of the process gas was varied.The values thus determined are shown in the following table:

Process gas p [bar] Hole diameter ø [μm] Conicity factor k 0.5approximately 131 approximately 0.25 1.0 approximately 152 approximately0.00 1.5 approximately 144 approximately 0.25 2.0 approximately 150approximately 0.30 2.5 approximately 145 approximately 0.90 3.0approximately 143 approximately 1.50 3.5 approximately 148 approximately1.90

These table values show that it is possible to produce very differenthole conicities and hole diameters only by varying the process gaspressure, i.e., with the same composition of the process gas.

With the example method according to the present invention, thecross-sectional shape of the bores/holes (e.g., conical or having aconstant cross section over the entire length) is adjustable in acontrolled manner. Melt burrs on the laser beam entrance side of theworkpiece and melt films on the hole walls may be reduced to a lowextent, hole edge shapes are adjustable, and remachining operations maybe greatly simplified or in the ideal case eliminated entirely. Thecriteria mentioned above in particular determine the quality of thehole, which is particularly high in holes produced by the device and/ormethod according to the present invention in comparison with holesproduced by known devices/methods. This example method may beparticularly suitable for producing precision microholes having adiameter of less than 250 μm, such as those provided in the nozzles offuel injector systems, for example. With the device according to thepresent invention and the method implementable with it, conical holeshaving a larger diameter are also producible with precision.

1. A method for producing holes in a workpiece, comprising: directing atleast one laser beam at a workpiece, wherein the at least one laser beamincludes a short-pulse laser beam or an ultrashort pulse laser beam;producing a hole having a desired conicity, a conicity factor of thehole being variable by optically influencing the laser beam; andsupplying at least one process gas to an interactive zone between thelaser beam and the workpiece, the process gas containing at least one ofhelium, oxygen, argon, nitrogen and a mixture thereof; wherein for adesired precision adjustment for increasing or reducing the conicity ofthe hole, the process gas is supplied to the interaction zone with apressure in a range from 0.1 bar to 20 bar and a process gas volume flowin a range from approximately 0.8 Nl/min to 270 Nl/min.
 2. The method asrecited in claim 1, further comprising: controlling at least oneparameter of the process gas during production of the hole.
 3. Themethod as recited in claim 2, wherein the controlling step includescontrolling the at least one parameter of the process gas overtime. 4.The method as recited in claim 1, wherein the hole has a diameter ofless than 250 μm.
 5. The method as recited in claim 4, wherein thediameter of the hole is less than 120 μm.
 6. The method as recited inclaim 1, wherein the process gas contains four different gases.
 7. Themethod as recited in claim 1, wherein the process gas is composed ofmultiple gases, and wherein the method further comprises: mixing thegases together before reaching the interaction zone.
 8. The method asrecited in claim 1, wherein the pressure is in a range from 0.3 bar to15 bar.
 9. The method as recited in claim 8, wherein the pressure is ina range from 0.5 bar to 10 bar.
 10. The method as recited in claim 1,wherein the process gas is supplied in a stream which runs coaxiallywith the laser beam.
 11. The method as recited in claim 1, wherein theprocess gas is supplied in a plurality of gas streams, a first one ofthe gas stream, being run coaxially with the laser beam, and a secondone of the gas streams being directed at an angle to the laser beam, theangle being in a range from 0° to 90°.
 12. The method as recited inclaim 1, wherein the process gas is supplied in a stream which runs from0° to 90° relative to the laser beam.